ASTERISM. Synonymous with constellation, a group of stars.
ASTERN. Any distance behind a vessel; in the after-part of the ship; in the direction of the stern, and therefore the opposite of ahead.—To drop astern, is to be left behind,—when abaft a right angle to the keel at the main-mast, she drops astern.
ASTEROIDS. The name by which the minor planets between the orbits of Jupiter and Mars were proposed to be distinguished by Sir W. Herschel. They are very small bodies, which have all been discovered since the commencement of the present century; yet their present number is over eighty.
ASTRAGAL. A moulding formerly round a cannon, at a little distance from its breech, the cascabel, and another near the muzzle. It is a half round on a flat moulding.
ASTRAL. Sidereal, relating to the stars.
ASTROLABE. An armillary sphere.—Sea-astrolabe, a useful graduated brass ring, with a movable index, for taking the altitude of stars and planets: it derived its name from the armillary sphere of Hipparchus, at Alexandria.
ASTROMETRY. The numerical expression of the apparent magnitudes of the so-called fixed stars.
ASTRONOMICAL CLOCK. A capital bit of horology, the pendulum of which is usually compensated to sidereal time, for astronomical purposes. (See [Sidereal Time].)
ASTRONOMICAL HOURS. Those which are reckoned from noon or midnight of one natural day, to noon or midnight of another.
ASTRONOMICAL OBSERVATIONS. There have been occasional slight records of celestial phenomena from the remotest times, but the most useful ones are those collected and preserved by Ptolemy. Since 1672, science has been enriched with a continued series of astronomical observations of accuracy and value never dreamed of by the ancients.